1. Field of the Invention
This invention is related to a method of producing semiconductor devices and, more particularly, to a preferable method of production to form MOSLSI on an SIMOX substrate.
2. Description of the Related Art
In general, a semiconductor substrate having a top single-crystal silicon layer, where device elements are formed, and a dielectric (SiO.sub.2) layer in a single-crystal silicon substrate is called as SOI (Silicon On Insulator) having two producing methods which are known well as a wafer bonding method and an oxygen ion implantation method. The former wafer bonding method is that the single-crystal silicon substrate is bonded on other single-crystal silicon substrate on which an oxide film is formed in advance and one of two single-crystal silicon substrates is polished to form a thin active silicon layer. The latter is called as SIMOX (Separation by IMplanted OXygen), in which high-dose oxygen ions (.sup.16 O.sup.+) are implanted into the single-crystal silicon substrate and a high temperature anneal processing (1,100.degree.-1,200.degree. C.) is practiced to cause Si and O to react, so that a buried oxide layer is formed in the silicon substrate. The thus method has been notable as it has been favorable to form the thin active silicon layer for large-scale integration lately.
The SIMOX substrate has, right after the oxygen ion implantation, there are both of compound Si-O with an unstable combining state besides chemical stable SiO.sub.2 in the SIMOX substrate right after the oxygen ion implantation, so that it has been said that a heat treatment at high temperatures, which is generally called as an anneal processing, after the ion implantation is required in order to form a dielectric layer having an abrupt interface. The above processing has a different way by ion implantation conditions, but in general, has the way in which the heat treatment is processed under an atmosphere containing therein 0.5-1.0% O.sub.2 added to Ar gas as an inert gas in 1,100.degree.-1,200.degree. C. for several hours.
It is known that, in the thus SIMOX substrate, structure and quality of the buried oxide layer formed under the top silicon layer by implanting the oxygen ions into the single-crystal silicon substrate depend on an ion implanting quantity (dose of ions), so that the dose of oxygen ions as 10.sup.17 -10.sup.18 /cm.sup.2 is required in order to form the buried oxide layer having the abrupt interface. But, there is disadvantages of producing crystal defects in the top single-crystal silicon layer when the dose of ions is increased, and observing the relation of a crystal dislocation density and the dose of ions, when the dose of ions reaches over 1.0.times.10.sup.18 /cm.sup.2, the dislocation density is increased and crystal quality of the top silicon layer where device elements are formed is inferior. When the dose of ions is controlled within the range between 0.5.times.10.sup.18 /cm.sup.2 and less than 1.0.times.10.sup.18 /cm.sup.2 in order not to produce the crystal defect, a breakdown electric field of the buried oxide layer is smaller and the breakdown voltage is decreased. Consequently, as a condition capable of obtaining the high breakdown electric field of the buried oxide layer and of achieving the low dislocation density in the top silicon layer, at an acceleration energy of 150-200 keV, for example, the ion implantation has been practiced by setting the dose of ions at around 0.4.times.10.sup.18 /cm.sup.2 (J. Mater. Res., Vol. 8, No. 3, 1993 pp. 524-534).
An MOS device is formed in the above-described SIMOX substrate by doping impurity into the top silicon layer dielectrically separated by the buffed oxide layer from the substrate silicon and forming a source and a drain. FIG. 5 is a schematic cross-sectional view of a CMOS device designed as described above. In the drawing, a symbol S means the source, a symbol D means the drain, a symbol G means a gate and a symbol V.sub.DD means the applied voltage. MOS-type LSI produced by employing the SIMOX substrate, where forms therein the buried oxide layer by implanting the oxygen ions into the single-crystal silicon substrate and practicing the anneal processing, has following disadvantages.
First, the high quality SIMOX substrate is that the dislocation density of the top single-crystal silicon layer is low and an .electrically insulated state of the buried oxide layer is excellent. However, since the dose of ions is defined in low level in order to restrain generation of the crystal defects, the thickness of the buried oxide layer become thinned down 80-90 nm and parasitic capacitance between the drain and the substrate silicon in an nMOS and between the drain and source and the substrate in a pMOS is larger, with the result that there has been a weak point as an operation speed of inverter is decreased.
Second, when the thickness of the buffed oxide layer is adjusted to be increased by increasing the dose of ions in order to attain smaller parasitic capacitance in producing of the SIMOX substrate, heat resistance in a direction of the thickness of the buried oxide layer is increased and the temperature of the device goes up, with the result that there has been weak points that negative electrical resistance property of drain electric current is serious and that saturated drain electric current is more and more decreased. Furthermore, when the thickness of the buried oxide layer is defined as for example 450 nm, there has been a weak point that a short-channel effect of MOSFET is serious (see IEEE 1991 IEDM Tech. Dig., 1991 pp. 675-678) and production of small sized MOSFET become difficult.
Third, as described above, the buried oxide layer is thinned down 80-90 nm in the high quality SIMOX substrate in which the crystal dislocation density is low and the electrically insulated state of the buried oxide layer is excellent, so that it is a weak point that high-voltage devices such as IGBT (Insulated Gate Bipolar Transistors) can not be fabricated on the thus SIMOX substrate. Consequently, the high-voltage devices have been fabricated on the substrate having the thick buried oxide layer of the SOI substrate produced by the wafer bonding method and so on, so that it has been difficult that both of the high-voltage devices and the low-voltage devices are formed together on the high quality SIMOX substrate.